The spin orbit coupling is a material property and also strong in the bulk. It is the bulk surface relation which gives rise to the band inversion at the surface and creates the particular surface properties of topological insulators. 

The spin orbit coupling is a material property and also strong in the bulk. It is the bulk surface relation which gives rise to the band inversion at the surface and creates the particular surface properties of topological insulators. 

Thank you sir for the answer.

But could you kindly elaborate more on the bulk surface relation in very simple language and if possible suggest some text for the same. Also, explain why TI material like bi2se3 shows for such kind of phenomenon and not other material like gold or anything not from TI list. I mean what is the basis for this phenomenon to occur.

I agree with the answer of Christian Binek. In fact, the spin-orbit coupling denotes an additional term in the Hamiltonian describing electrons in the periodic potential of the nuclei (see e.g. Ref. [1, Eq. (1.1)]). This term has a relativistic origin and is particularly relevant for materials containing heavy atoms such as bismuth. In topological insulators, the spin-orbit coupling may lead to a spin splitting of the electronic surface (or edge) states. Note that also gold shows a spin splitting of surface states [2], although this is not a topological effect. Furthermore, there are indeed materials which show a spin splitting of the bulk energy bands. The most prominent example in this respect is BiTeI, whose bulk energy bands can be effectively described by the Rashba Hamiltonian. Here, the spin splitting mainly results from the spin-orbit coupling of Bi atoms and the non-centrosymmetric crystal structure of this material (see e.g. [3, 4]).

[1] R. Winkler, Spin-orbit coupling effects in two-dimensional electron and hole systems (Springer, 2003).

[2] S. LaShell, B.A. McDougall, E. Jensen, Phys. Rev. Lett. 77, 3419 (1996).

[3] K. Ishizaka et al., Nat. Mater. 10, 521 (2011).

[4] S. Schwalbe et al., https://www.researchgate.net/publication/305615774_Ab_initio_electronic_structure_and_optical_conductivity_of_bismuth_tellurohalides

Article Ab initio electronic structure and optical conductivity of b...